The present invention refers to a process for the purification of melamine manufactured by synthesis from urea, according to a high pressure, non-catalytic process, and more particularly a process which is able to transform certain impurities, resulting from melamine synthesis or subsequent manipulations, essentially into melamine.
It is well known that in the high pressure melamine synthesis from urea a series of by-products is formed which affects melamine purity.
Particularly in the reactor, during melamine synthesis, and subsequently in the containers wherein melted melamine resides at a temperature higher than 350xc2x0 C., typical melamine deammoniating condensation products are formed in a large amount. This category of condensation products are hereinafter collectively referred to as polycondensates. The simplest product is melam resulting from two melamine molecule condensation and one ammonia molecule separation. 
From two melamine molecules another product can be formed by separation of two ammonia molecules: 
Another by-product resulting from melamine deammoniating condensation is melon: 
Generally liquid melamine (and therefore at a temperature higher than melting temperature of 350xc2x0 C.), both inside reaction vessel and during the melamine residence in downstream treatment equipment, is subject to a chemical condensation associated with ammonia separation resulting in the formation of abovementioned products or even more complex ones collectively referred to as polycondensates. Polycondensates formation increases when ammonia partial pressure is lower and residence time of melted melamine is longer. These products are undesired for melamine use in the manufacture of formaldehyde/melamine condensation resins, therefore in the final melamine product a polycondensates content lower than 1000 ppm is requested as a minimum acceptable content.
Fortunately the reactions leading to these by-products are equilibrium reactions. Therefore melamine polluted by polycondensates may be purified, by subjecting said melamine to an ammonia action under suitable temperature and pressure conditions. Then, the ammonia action not only results in a polycondensates elimination, but due to the polycondensate transformation into melamine, an increase of global yield is also obtained.
However the operative conditions necessary to reach the above results are quite heavy and expensive in that they imply to subject melted melamine to an ammonia partial pressure higher than 250 bars (2.5xc3x97107 Pa), preferably above 400 bars (4xc3x97107 Pa), in order to obtain a melamine of an acceptable purity.
On the other hand the transformation of polycondensates to melamine in an aqueous medium requires less severe and less expensive operating conditions. In the industrial practice all melamine synthesis processes include a recovery and purification step of raw melamine coming out of the synthesis reactor in an aqueous medium.
Said process consists in sending the reaction product, comprising a raw melamine liquid phase and an essentially NH3xe2x80x94 and CO2xe2x80x94 containing gas phase, to an appropriate contacting equipment (quench tower) wherein the product is cooled down by means of water. Pressure is simultaneously reduced from reaction pressure (higher than 70 barsxe2x88x927xc3x97106 Pa) to a pressure of about 25 bars (2.5xc3x97106 Pa) and at a temperature lower than 165xc2x0 C. to limit both corrosion problems and the amount of steam entrained in vapour phase.
As a result of the above contact, a gas phase containing NH3, CO2 and steam and a liquid aqueous phase which contains melamine, polycondensates, unreacted urea as well as minor impurities are obtained. From the liquid phase, melamine is recovered, through several manipulation and crystallisation steps.
Typically, said solution has a melamine concentration of 5 to 15% w, and contains also some unreacted urea, dissolved gases as NH3 and CO2 as well as other minor impurities including the polycondensates. The latter are present in the amount of 1:30 to 1:70 with respect to dissolved melamine.
By subjecting this solution to ammonia action, the polycondensates transformation into melamine takes places; however, such reaction proceeds with a very low rate requiring a very long contact time to reach a polycondensates transformation degree matching the required purity specifications.
For this reason in the commercial plants, the liquid solution coming out of the quenching tower is kept as such, with or without any ammonia addition for a suitable period of time which may be of one to several hours. Said technique requires the use of large treatment volumes that means, taking into consideration the temperature and pressure conditions and the corrosive characteristics of the products under treatment, the use of special stainless steels, high apparatus thickness and, therefore, higher investment costs. Moreover in the course of the above treatment an undesirable loss of product takes place because of the contemporaneous melamine hydrolysis reaction in presence of water, which loss is higher, the longer is the treatment time necessary to reduce the polycondensates to an acceptable value.
There is not, at this moment, any available melamine purification process capable to reduce the impurity contents, particularly the by-products originated from the melamine deammoniating condensation, which do not require a melamine treatment under severe temperature and pressure conditions or capable to reduce the reaction time to an economically acceptable value. The present invention intends to solve the abovementioned problems.
It has been surprisingly found that the conversion rate of polycondensates to melamine in an aqueous environment in presence of ammonia, which, as we saw, is very low, may considerably increase if the CO2 content of the aqueous solution deriving from the water quenching of the reaction products is reduced to a value lower than 0.5 and preferably lower than 0.2% wt.